2015-2016 Colloquia Abstracts

Fall 2015

Review of Requirements and Timelines for Physics and Astronomy Graduate Students

Requirements and procedures for graduate students in the Department of Physics and Astronomy have been significantly revised over the last few years. While the resulting system reduces the the number of required courses to be more in line with national norms, the transition requires an effort to institute best practices for how to complete requirements in a timely manner. This talk and discussion is intended to help clarify requirements and expectations for both new and continuing graduate students. Many of these requirements and expectations have specific ramification for the completion of degree or the continuation of financial support. Forms submitted at the various stages in order to document completion of requirements will also be discussed with the intent of helping students avoid complications on the way to their degree.

All full-time Physics and Astronomy graduate students are required to attend. Faculty are welcome to attend if they have questions or would like more information about the structure or implementation of the graduate curriculum.

New magnetic materials promise advancement in a variety of fields: spintronics, nonvolatile logic devices, power generation/conversion, chemical sensing. However, much of the investigation space for simple alloys and oxides is explored, and continued progress requires us to investigate more complex material systems. Although complexity promises tailor-made solutions, a major challenge lies in lowering defect and disorder levels to the point that the intrinsic material physics appears.

In this talk, I will outline our methods to create and characterize new complex functional materials and heterostructures. Thin film growth will be achieved by a custom-built sputter system, combining the compositional control via combinatorial deposition with the wide element choice and low energetics of off-axis sputtering. Conventional characterization will be combined with powerful x-ray synchrotron techniques (XAS, XMCD, RIXS) and tunnel-device analysis (IETS, MTJ), to obtain a fuller understanding of the underlying physics of each new material.

Faculty Interests Mini-Colloquia: Quantum Matter in Low-Dimensions

I will offer an overview of my research in condensed matter theory on low-dimensional systems with beginning graduate students in mind. What is it like to do research in condensed matter theory? What do we learn from such research and what kinds of tools do we use? What are the forefront research topics? I will try to answer these questions in this mini-colloquium.

Roughly 1/1000 of nearby active galactic nuclei (AGN), and a larger fraction of HI-selected AGN, are accompanied by extended emission-line regions on scales 10 kpc and larger (beyond the normal interstellar medium of the host galaxy). In about 40% of these, the AGN falls short of the energy budget needed to photoionize these clouds by 1-3 orders of magnitude, implying that the AGN have faded over the relevant light-travel times. All these are in interacting or post merger systems, possibly needed for sufficient distant, cold H I to trace the AGN’s ionization history.In many of the fading candidates, low abundances and quiescent kinematics indicate that we see photoionized tidal debris rather than outflowing material; separate signatures of sub-kiloparsec outflows are seen in some cases, giving an intriguing hint that the mode of energy output incidental to accretion may be changing and not just the accretion rate itself. Light curves derived from recombination balance show e-folding timescales from centuries to a few millenia, short compared to simple accretion-disk expectations. These rapid, large-amplitude changes may be associated with SMBHs in binary systems or with local feedback processes. These results suggest that the demographics of accreting supermassive black holes are broader than derived from ongoing AGN signatures alone, supported by new samples of analogous objects in both very nearby and moderate-redshift systems.

Resolved galaxy studies with Sloan Digital Sky Survey IV

I will give a broad overview of my different areas of research in the field of galaxy evolution and describe the Mapping Nearby Galaxies at APO (MaNGA) survey, a project which is part of Sloan Digital Sky Survey 4 (SDSS IV). Unlike previous SDSS surveys which measured spectra only at the centers of target galaxies, MaNGA bundles sets of optical fibers into tightly-packed arrays, enabling spectral measurements across the face of each of ~10,000 nearby galaxies. I will present some early results from the survey.

Holography near and far from equilibrium

Over the past 20 years an exciting new research area has emerged in Physics. It brings together physicists studying string theory, heavy ion collisions, condensed matter systems, and many more. What unifies all of these subjects is the question: how do quantum systems behave at strong coupling and far from equilibrium? The technical connection between these different subjects is provided by a holographic correspondence: strongly coupled quantum systems on one side correspond to certain theories of gravity on the other side. In this mini-colloquium I will provide an intuitive introduction to the fascinating concepts of this thriving research area.

The restart of the CMS data taking at the LHC and the activities of the UA CMS group

After two years of maintenance and upgrade work, the Large Hadron Collider (LHC) and the CMS Experiment have recently restarted data taking using proton-proton collisions at the energy of 13 TeV, almost twice as much as the highest energy reached during Run-1 (2009-2012). The new data will be used to continue the study of the Higgs Boson and explore another uncharted energy range to search for new physics. A brief overview of the CMS experiment and the research activities of the CMS group at the University of Alabama is given.

Understanding the physics of galaxy clusters

Recent years have witnessed the emergence of galaxy clusters as a fundamental cosmological and astrophysical tool. In this talk I will review recent advances in the physics of clusters. I will focus on what can be learned from X-ray, Sunyaev Zel’dovich and lensing observations, and hydrodynamical numerical simulations. I will discuss some recent developments in the modeling of both dark matter and the intracluster medium, in particular with respect to the first determination of the intrinsic shape and physical parameters of both the dark matter and intracluster medium in triaxial galaxy clusters via a multiwavelength analysis. Next, I will discuss a method to determine the physical parameters in clusters, including gas temperature and peculiar velocities, via multi-frequency Sunyaev Zel’dovich observations and independent of X-ray observations. Finally, I will review recent developments in the virialization region of clusters, which has recently gained a lot of attention in the scientific community in offering a direct view of structure formation. In particular, I will present novel results on the physical properties of the intracluster medium at the cluster outskirts, including average emission measure, gas density and gas fraction, temperature, entropy and gas inhomogeneities.

Building a Holographic Model of the Kondo Effect

I will give a brief overview of the Kondo effect, and then show how one can arrive at a dual model of the Kondo effect via gauge/gravity duality. The model is a useful first step towards studying many open problems involving impurities, including for example the Kondo lattice problem.

Dark Matter Halo Shapes in Cosmological Simulation

During the past two years I have utilized the MUGS cosmological galaxy formation simulations to explore the processes of galaxy formation and evolution. I will introduce this simulation suite and outline one aspect of my research in Alabama. Using all sixteen galaxies in MUGS I identified persistent structure in the shape profile of the dark matter halos of a substantial fraction of these galaxies. This shape feature, which lies close to the traditional but much maligned “virial radiusa,” marks the interface between the dark matter halo and the large-scale environment.

Next Questions in Neutrino Physics and the NOvA Experiment

The discovery of neutrino mass in 1998 spawned a world-wide effort to better understand neutrino properties using neutrinos from the Sun, the atmosphere, reactors, and from accelerators. While much has been learned since then, several important questions remain: which neutrino is heaviest? Is there a symmetry in neutrino mixing? Do neutrinos break matter/antimatter symmetry? Is the framework we use to understand neutrinos complete or is there more? The NOvA experiment is designed to address each of these remaining questions and has recently completed construction and begun operations. I will summarize the important factors that guided the NOvA design, show some highlights from construction, and present the first results from measurements of muon neutrino disappearance and electron neutrino appearance from the experiment.

The band theory of crystalline solids provides the fundamental basis for understanding materials and phenomena. It is generally believed that for most physical applications the band dispersion alone carries sufficient information to give proper account of various thermodynamic and transport properties. Recently, this belief is challenged by the realization that the Berry phase of the electronic wave function can also have a profound effect on material properties and is responsible for a spectrum of quantum phenomena. In this talk, I will review the basic concept of the Berry phase in crystals, followed by its application in a number of interesting phenomena, such as the anomalous Hall effect and valley-dependent phenomena. Given its broad range of applications and essential role in understanding these phenomena, it is clear that the Berry phase should be included as a basic ingredient in the electron theory of materials.

Black holes vs quantum mechanics: who is winning?

Forty years ago, Bekenstein and Hawking famously showed that, according to the rules of quantum mechanics, black holes have a finite temperature and entropy. This remarkable discovery revealed a fundamental clash between Einstein’s theory of General Relativity and core principles of quantum information theory. This puzzle, known as the black hole information paradox, or (more vividly) as the firewall paradox, is still leading to intense discussion and disagreement. In this colloquium I summarize the current status of this debate and highlight some lessons that have been learned.

The Formulation of General Relativity

The year 2015 marks the centenary of Einstein’s formulation of the general theory of relativity. There are three major ingredients that underlie general relativity: The first is the special theory of relativity—formulated by Einstein in 1905—which demonstrated that space and time are intimately related. In special relativity, space and time comprise a single entity, “spacetime,” and the space and time relationships between events are described by a certain “geometry.” The second key ingredient is the equivalence principle, which simply says that all bodies fall the same way in a gravitational field. Although this fact was well known since the time of Galileo, it suggested to Einstein that the effects normally attributed to gravitational forces actually correspond to a curved spacetime geometry. The final major ingredient was to obtain the equation that relates the curvature of spacetime to the matter content in the universe. It is here that Einstein struggled and went badly off track for several years, but in a remarkable series of papers in November, 1915, he succeeded in completing this step. The resulting theory, general relativity, stands as one of the most remarkable intellectual achievements to ever have occurred in science.

Black Hole Horizons

Black holes were discovered as solutions of Einstein’s equations a century ago, but we still do not have a complete understanding of them as real objects. We still do not have conclusive evidence for the presence of an event horizon as a black hole’s point-of-no-return surface. The event horizon of the black hole at the center of our galaxy is probably 15 million miles across, but it is 27,000 light-years away. To image it at sub-millimeter wavelengths, a telescope as large as the Earth is required. A worldwide network of radio telescopes can obtain the required resolution. Using such a network, we hope to see how radio waves from the black hole’s surroundings are bent and absorbed, just as in Christopher Nolan’s movie Interstellar. We should be able to observe a kind of central “shadow.” By comparing the size, shape and sharpness of this shadow with theoretical predictions, we can test general relativity. If the shadow is, say, half as big (or twice as large) as predicted, general relativity cannot be correct.

Characterization of Magnetic Nanostructures for Spintronics with Polarized Neutron Scattering

Probing the spin structure in thin films and nanostructured materials via Polarized Neutron Reflectometry (PNR) is a type of experiments addressing not only a kind of surface visualization or integration over the whole sample size. PNR is a tool providing depth resolved magnetometry that requires high intensity with a low background and a high polarization of the neutron beam. In this talk I will make an introduction to PNR and present the Magnetism Reflectometer (MR) at the Spallation Neutron Source. The spectrum of experiments performed at MR covers multiple scientific areas, e.g. the phenomenon of exchange coupling in topological insulator-ferromagnetic insulator heterostructures [1], in-situ annealing and Boron diffusion in perpendicular magnetic tunnel junction [2], direct evidence of anomalous interfacial magnetization in metamagnetic Pd doped FeRh thin films [3], magnetic properties in oxide heterostructures [4, 5]. I will focus on the selected topics and present results of recent experiments.

Research at Oak Ridge National Laboratory’s Spallation Neutron Source was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy.

Uncovering the physics and chemistry of complex oxide surfaces

Complex oxides with perovskite structure attract a lot of interest due to their superior catalytic activity and sensory properties. For example, perovskite oxides have been recently proposed as cost-efficient replacements for the expensive noble-metal catalysts that are currently used in solid-oxide fuel cells for oxygen reduction. These properties are inherently connected to their surfaces. Perovskite surfaces also provide fertile ground for the discovery of novel electronic and magnetic phenomena. However not much is known about the surface structure of perovskites due to their chemical complexity. In this work, I will discuss results combining scanning transmission electron microscopy (STEM) imaging and electron energy loss spectroscopy (EELS) experiments with density functional theory (DFT) based calculations to reveal the physics and chemistry of the surfaces of two distinct perovskite systems.

In the first part of the talk, I will show the application of STEM EELS experiments in conjunction with DFT calculations to understand the surface reconstruction of a complex ferrite superlattice. I will show that the surface is terminated with FeO4 tetrahedra instead of the FeO6 octahedra as present in the bulk. This surface reconstruction results in an exotic phase where the surface layer displays a half-metallic ferromagnetic behavior, while the bulk remains antiferromagnetic and insulating, similar to the class of topological insulators.

In the last part of the talk, I will discuss the implications of surface termination on the two-dimensional electron gas (2DEG) formed at the interface of two insulating oxides SrTiO3 and LaAlO3. In these films, the experimentally observed 2DEG density is almost an order of magnitude lower than the theoretically predicted value, a topic that has been under considerable debate for over a decade. I will show that the surface reconstruction of the films plays a crucial role in the experimentally observed lower 2DEG density.

Overall, the results provide insights into complex oxide surfaces that can be used as a new route to engineer their properties.

The Persistence of Memory: What Supernova Remnants Can Tell Us about Type Ia Supernova Progenitors

Despite decades of continuing observational and theoretical efforts, the identity of the progenitor systems of Type Ia Supernovae (SN Ia) remains obscure. Recent results have added to the controversy about the nature of the binary companion of the exploding white dwarf, which must be either another white dwarf (double degenerate systems, DD) or a non-degenerate star (single degenerate systems, SD). On the one hand, there are no clear signs of dynamical interaction between SN ejecta and circumstellar material, which seems to favor DD systems. On the other hand, there is mounting evidence that at least some exploding SN Ia have ejecta masses very close to the Chandrasekhar limit, which is more naturally explained by the SD scenario. I will describe recent X-ray observations of Type Ia Supernova Remnants that can shed light on the properties of SN Ia progenitors and the SD vs. DD debate.

Spring 2016

Review of Timeline and Requirements for Physics and Astronomy Graduate Students

With the aim of helping keep students on track, I will briefly review the expected timeline for progress through our Ph.D. program as well as the requirements for maintaining financial support and eligibility for degree. The roles and timeline of various forms will be discussed in detail.

“Just a Minute” Student Talks

Abstract: Research, in the form of a dissertation project, is the most important component of a Ph.D. student’s graduate career. In order to help and encourage junior graduate students to make their transition into
research, all current Ph.D. candidates (students who have passed their prelim and have an approved dissertation topic) will give a brief 1-2 minute summary of their dissertation subject.

UA Benefits

Our employer, UA, does offer a variety of benefits to its employees. Some come automatically, while for others the employee has to sign up. Some we hopefully might never use while others are crucial. However, not all crucial components are fully utilized. For example, one third of eligible employees does not make use of the employer contribution matching to the 403(B) savings plans. I will discuss UA benefits, with an emphasis on retirement benefits.

Abstract: Itinerant ferromagnetism (FM) is intrinsically a strongly correlated phenomenon, which remains a major challenge of condensed matter physics. Most FM materials are orbital-active with prominent Hund’s coupling. However, the local physics of Hund’s rule usually does not lead to the FM long-range order. Furthermore, the magnetic phase transitions of itinerant electrons are also long-standing problems difficult to handle by using perturbative methods. In this talk, I will present non-perturbative studies on itinerant FM. Exact theorems are established for a stable itinerant FM phase in a large region of electron densities in multi-orbital systems, which provide sufficient conditions for Hund’s rule to build up global FM coherence. In addition, thermodynamic properties and magnetic phase transitions of itinerant electrons are studied via sign-problem-free quantum Monte Carlo simulations at generic fillings. Without introducing local moments as a priori, the Curie-Weiss metal behavior is identified in a wide range of temperatures. These results will provide important guidance to the current experimental search for novel itinerant FM states in a large class of systems ranging from the transition-metal-oxide heterostructures (e.g. LaAlO3/SrTiO3) to the p-orbital bands in optical lattices filled with ultra-cold fermions.

Transport of charge and heat in disordered many-body systems

The influence of disorder on interacting many-body systems plays a prominent role in modern condensed matter physics. Not only is the presence of disorder often unavoidable in complex materials, disorder leads to fascinating phenomena in their own right. In this talk, I explore the interplay of disorder and interactions with two examples.

In recent years, progress in fabrication and measurement techniques has lead to detailed experimental studies of fluctuation phenomena in low-dimensional superconductors. I discuss how superconducting fluctuations can increase the resistance of thin disordered films which are on the verge of becoming superconductors. In our theoretical studies on this subject we deviate from the traditional diagrammatic approach to the problem, which lead to conflicting results, and obtain results for the fluctuation conductivity in the entire normal-metal part of the phase diagram [1]. In the second part of the talk I present elements of our theory of thermal transport in the disordered electron liquid [2]. In particular, I address the fate of the Wiedemann Franz law at low temperatures and describe our prediction for the thermal conductivity on the metallic side of the metal-insulator transition in SiMOSFETs.

LIGO-themed mini colloquia: A search for neutrinos in coincidence with the first gravitational wave event

The detection of the first gravitational wave (GW) event by LIGO, a hundred years after Einstein’s general relativity theory suggested their existence, represents one of the greatest scientific breakthroughs of recent years. For neutrino hunters, such as IceCube and ANTARES, this discovery is also a great step forward for the nascent field of multimessenger astronomy. Here I discuss IceCube’s search for neutrinos in coincidence with LIGO’s first gravitational wave detection.

The inspiral and merger of binaries consisting of BHs or neutron stars (NSs) have been discussed as the primary source for ground-based GW interferometers for many decades. Here I discuss the expectations for the binary black hole masses and rates from stellar formation channels, how the recent LIGO event GW150914 relates to and constrains these, and some prospects for what can be learned in the near future as more events are observed.